g_level.h
------------------
struct FLevelStats
{
	FString MapName;

	int			maptime;		// time in the map
	int			totaltime;		// time in the game

	int			total_secrets;
	int			found_secrets;

	int			total_items;
	int			found_items;

	int			total_monsters;
	int			killed_monsters;
};

void LevelLocalToStat(FLevelStats& stat, FLevelLocals& level);

g_level.cpp
------------------
TArray<FLevelStats> levelstats; //level statistics since new game has started

void LevelLocalToStat(FLevelStats& stat, FLevelLocals& level)
{
	stat.MapName = level.MapName;
	stat.MapName.ToLower();
	stat.maptime = level.maptime;
	stat.totaltime = level.totaltime;

	stat.total_secrets = level.total_secrets;
	stat.found_secrets = level.found_secrets;

	stat.total_items = level.total_items;
	stat.found_items = level.found_items;

	stat.total_monsters = level.total_monsters;
	stat.killed_monsters = level.killed_monsters;
}

void G_NewInit ()
{
	...G_NewInit code
	
	levelstats.Clear();
}

void G_ChangeLevel(const char *levelname, int position, int flags, int nextSkill)
{
	FSerializer arc;

	if (arc.OpenWriter())
	{
		Renderer->StartSerialize(arc);

		FLevelStats stat;
		LevelLocalToStat(stat, level);
		levelstats.Push(stat);

		arc.BeginArray("levels");
		unsigned int count = levelstats.Size();
		for (unsigned int i = 0; i < count; i++)
			Serialize(arc, nullptr, levelstats[i]);
		arc.EndArray();

		Renderer->EndSerialize(arc);

		FCompressedBuffer buffer = arc.GetCompressedOutput();

		TArray<FString> filenames;
		TArray<FCompressedBuffer> content;
		filenames.Push("maps.json");
		content.Push(buffer);

		WriteZip("stats.zip", filenames, content);
	}

	....G_ChangeLevel
}

serializer.h
------------------
FSerializer &Serialize(FSerializer &arc, const char *key, FLevelStats &stats);

d_protocol.cpp
------------------
FSerializer &Serialize(FSerializer &arc, const char *key, FLevelStats &stats)
{
	if (arc.BeginObject(key))
	{
		arc("MapName", stats.MapName)
			("found_secrets", stats.found_secrets)
			("found_items", stats.found_items)
			("killed_monsters", stats.killed_monsters)
			("total_secrets", stats.total_secrets)
			("total_items", stats.total_items)
			("total_monsters", stats.total_monsters)
			("maptime", stats.maptime)
			("totaltime", stats.totaltime)
			.EndObject();
	}

	return arc;
}